1. Field of the invention
The present invention relates to a deaeration tank.
2. Description of the related art
Conventionally, in treating wastewater, i.e. object water of treatment (hereinafter merely referred to as object water), an organic coagulant is added to object water contained in a coagulating sedimentation tank so as to coagulate and settle suspended solids (SS) into sludge, which is then removed. However, when fine bubbles contained in object water adhere to the thus-formed sludge, the sludge does not settle but is suspended in the object water. As a result, the suspended sludge flows out from the coagulating sedimentation tank into a subsequent treatment tank.
Thus, in order to remove fine bubbles from the object water, various pretreatments are performed before the object water is subjected to coagulating sedimentation. Such pretreatments include a coarse bubble type deaeration method and a mechanical stirring method.
FIG. 1 is a schematic sectional view showing a conventional deaeration tank which employs the coarse bubble type deaeration method.
In FIG. 1, reference numeral 11 denotes a cylindrical or square tank body, and numeral 12 denotes a partition for dividing the tank body 11 into a deaeration section 21 and a discharge section 22. A communication opening 23 is formed between the lower end of the partition 12 and the bottom wall 11a of the tank body 11 to thereby allow communication between the deaeration section 21 and the discharge section 22. Also, a line L1 is connected to the deaeration section 21 at an upper position thereof so as to feed object water into the deaeration section 21, while a line L2 is connected to the discharge section 22 at an upper position thereof so as to discharge deaerated the object water into an unillustrated coagulating sedimentation tank. An inorganic coagulant is added to the deaeration section 21, so that polarity of SS contained in the object water is neutralized.
A nozzle 13 for injecting air into the deaeration section 21 is disposed within the deaeration section 21 in the vicinity of the bottom wall 11a. A line L3 is connected to the nozzle 13 in order to feed air thereto. A plurality of injection ports 13a are formed in the nozzle 13.
In the thus-structured deaeration tank, when object water is fed into the deaeration section 21 through the line L1, and air is fed to the nozzle 13 through the line L3, the fed air is injected into the deaeration section 21 through the injection ports 13a of the nozzle 13. Thus-injected air ascends in the object water in the form of coarse bubbles and is then released from the surface of the object water.
During this ascent of coarse bubbles within the deaeration section 21, through contact with the object water, coarse bubbles attract fine bubbles contained in the object water, so that the object water is deaerated. Thus-deaerated object water descends within the deaeration section 21 and then enters the discharge section 22 through the communication opening 23. Then, deaerated object water ascends within the discharge section 22 and is discharged into the coagulating sedimentation tank through the line L2.
However, in the above-described conventional deaeration tank, when the amount of air which has failed to become coarse bubbles is relatively large, some of the fine bubbles originally contained in object water remain intact without being attracted by coarse bubbles. As a result, the object water is not deaerated sufficiently.
Also, air which has failed to become coarse bubbles ascends in the object water at a speed slower than that of the object water descending within the deaeration section 21. Accordingly, such air, together with fine bubbles contained in the object water, descends within the deaeration section 21 following the descending flow of the object water, then enters the discharge section 22 through the communication opening 23, and is finally discharged into the coagulating sedimentation tank through the line L2.
To solve the problem, the distance between the bottom wall 11a and the nozzle 13 is made at least 500 mm longer than the distance between the bottom wall 11a and the lower end of the partition 12, or the feed rate of object water into the deaeration section 21 is reduced so as to reduce the descending speed of the object water, thereby preventing air and fine bubbles from entering the discharge section 22 together with the object water through the communication opening 23.
However, when the distance between the bottom wall 11a and the nozzle 13 is made at least 500 mm longer than the distance between the bottom wall 11a and the lower end of the partition 12, the size of the deaeration tank must be increased accordingly. Also, when the feed rate of the object water into the deaeration section 21 is reduced, the treatment capability of the deaeration tank decreases accordingly.